LONDON (Reuters) – Prime Minister Theresa May’s control of the Brexit process will undergo its stiffest parliamentary test yet on Wednesday, when she faces a showdown with rebels in her own party over the laws that will take Britain out of the European Union.

May’s government is trying to pass a bill through parliament that will repeal the 1972 legislation binding Britain to the EU and copy existing EU law into domestic law to ensure legal continuity after ‘Exit Day’ on March 29, 2019.

After six days of debate which has jumped between the legal minutiae of Brexit and the gaping ideological differences between ‘Remainers’ and ‘Leavers’, May could face an embarrassing parliamentary defeat as lawmakers demand a greater say over the final exit deal.

In June, she gambled on a snap election to strengthen her party’s majority in the 650-seat parliament but instead bungled her campaign and ended up with a minority government propped up by the 10 votes of a small, pro-Brexit Northern Irish party.

Since then she has struggled to assert her authority over a Conservative Party which is deeply divided over the best route out of the EU.

Wednesday’s likely flashpoint is an amendment to the bill put forward by a member of May’s own party, the government’s former attorney general, Dominic Grieve.

If passed by a simple majority vote, the proposal would require parliament to approve the government’s final Brexit deal by passing a separate written law once the terms of the withdrawal agreement are known. That could allow lawmakers to send May back to the negotiating table.

“I don’t see any possibility of my backing down on this at all … One has to stand up for one’s principles,” Grieve told BBC radio, adding that he thought he had enough fellow rebels to overturns May’s slim working majority in parliament.

The government initially proposed giving lawmakers a vote on the Brexit deal which would give them the choice to accept a negotiated settlement or walk away without a deal.

It has since conceded that a separate piece of legislation, allowing members of parliament (MPs) more say on the deal, would be necessary. The government is planning to pass another bill once the final Brexit deal with Brussels is agreed which will implement the terms of the withdrawal agreement.

However, that has not yet been enough to head off Grieve, who says the current wording of the bill did not give enough assurances on the topic.

The government has been forced to give ground on several issues to ward off other rebellions over the Brexit laws.

On Monday, it accepted a proposal to allow lawmakers greater scrutiny over the passage of EU law into British law.

Still, after striking a deal with Brussels last week to move exit negotiations on to the next phase, covering trade and transition arrangements, May won approval from both the remain and leave factions of her party, suggesting attempts to unseat her were on hold.

The government has not ruled out making last-minute concessions to appease Grieve and the 20 or so Conservative rebels who could join forces with the opposition to inflict defeat.

“I think what the MPs are looking for is clarity … We’re looking at the amendment and will respond in due course,” May’s spokesman said on Tuesday.